The present invention relates to polycarbonate plastics useful for lenses or other articles and particularly to allyl carbonate ophthalmic lenses and lens components exhibiting improved hardness and scratch resistance.
The most widely used plastic for the production of plastic ophthalmic lenses is an allyl carbonate plastic, specifically diethylene glycol bis(allyl carbonate) plastic which is well known and commercially available in monomer form as CR-39.RTM. monomer. The hardness and scratch resistance of lenses made of polymerized CR-39.RTM. monomer depend on several factors, including the composition and concentration of the polymerization catalyst used, and the thermal treatment employed in polymerizing the resin to obtain the solid product. The polymerization catalyst most commonly employed by CR-39.RTM. lens manufacturers is isopropyl percarbonate, typically at a concentration of about 3% by weight in the resin-catalyst mixture to be hardened.
Commercial lens manufacturers frequently utilize hardness testing to control the quality of their product, since hardness testing is simpler than most scratch testing procedures and correlates well with scratch resistance in cured CR-39.RTM. plastic lenses. The hardness values exhibited by commercially available lenses, as measured by a standard Barber-Colman impresser device (Barcol GYZJ 935 hprocedures and correlates well with scratch resistance in cured CR-39.RTM. plastic lenses. The hardness values exhibited by commercially available lenses, as measured by a standard Barber-Colman impresser device (Barcol GYZJ 935 hardness tester), will typically range from about 77-80.
It has recently been proposed to use allyl carbonate resins to produce lightweight glass-plastic laminated lenses by the direct casting of these resins against adhesive-coated glass sheet. As disclosed in a copending, commonly assigned application by S. T. Gulati et al., Ser. No. 18,107 filed Mar. 7, 1979, now U.S. Pat. No. 4,268,134 a typical design for such a lens incorporates a thin glass core element composed of photochromic (reversibly darkenable) glass microsheet positioned between two opposing plastic surface elements. The plastic surface elements are formed by the direct casting of CR-39.RTM. plastic resin against the glass, with the glass being first coated with a polymeric bonding adhesive providing a bonding layer between the glass and plastic after curing.
In grinding the surfaces of such direct-cast laminated lenses to selected prescriptions for ophthalmic use, it was recently discovered that the hardness of the plastic surface elements, although apparently adequate as measured at the surfaces of the lenses as made, was well below commercially acceptable levels in a boundary region of the plastic layers adjacent to the adhesive bonding layers in the laminates. For example, at plastic layer thicknesses of 1 mm or less, Barcol hardness values of 71 and below were frequently observed. Since it was anticipated that prescription grinding would frequently involve the removal of sufficient material from the outer plastic elements to approach this thickness, thus exposing the softer plastic of the boundary region, a solution to the problem of low hardness adjacent the boundary region was sought.